Approximately 19,000 children are born to epileptic women in the United States annually. Fetal hydantoin syndrome (FHS) is marked by pre-and postnatal growth deficiency, distal phalangeal/nail hypoplasia, and facial stigmata occurring in 10-11% of children exposed in utero to the widely used anticonvulsant phenytoin (Dilantin). Fetal hydantoin effect (FHE) occurs in 30% of exposed children who have some, but not all, of the FHS features. In addition, phenytoin exposure in the first trimester is associated with increasing four times the incidence of oral clefting and congenital heart defects in humans. Microsomal epoxide hydrolase (mEH) detoxicates the bioactive arene oxide metabolite of phenytoin. Low mEH activity in fibroblasts and amniocytes has been associated with FHS, including two children who had an oral cleft. The purpose of this study is to test for an association between mEH polymorphisms, low mEH activity and an adverse outcome following phenytoin exposure during gestation. To test the hypothesis that patients with FHS or FHE have a relatively low mEH activity as compared to non-affected siblings, affected children, siblings and parents will have their blood drawn. Leukocytes will be isolated for determination of mEH activity by multiple assays and HPLC. ANOVA will be used to test at p less than or equal to 0.05 that the mEH activity means of exposed affected and exposed non-affected persons are different. DNA will be isolated from the blood samples, the 9 mEH exons amplified by polymerase chain reaction (PCR) and screened for polymorphisms by single strand conformation polymorphism (SSCP) analysis. Two candidate mutations are an mEH allele (Y113 yields H in exon 3) associated with approximately 40% less mEH activity compared to the most common human allele, and an mEH allele (R338 yields C in exon 7) associated with 60-65% less mEH activity, and increased heat lability of the enzyme activity in mice. If the mutation screening is positive, the exon will be sequenced. Contingency table testing will be used to test if the relative frequency of mEH polymorphisms are different between exposed affected and exposed non-affected persons at p less than or equal to 0.05. Confirmation of the hypothesis that certain mEH polymorphisms related to low mEH activity are associated with adverse outcome following phenytoin exposure in utero, demonstration of a vertical transmission of mEH polymorphisms and activity from parents to children, and identification of an mEH genetic marker(s) for low mEH activity associated with susceptibility to phenytoin-associated major and minor anomalies will allow identification of families and individuals at greater risk. This may led to the consideration of alternative maternal anticonvulsant therapeutics.